1. Field of the Invention
The present invention relates to a piezoelectric element having a multilayered structure (multilayered piezoelectric element) to be used as ultrasonic transducers, piezoelectric actuators, and so on, and relates to a method of manufacturing the multilayered piezoelectric element.
2. Description of a Related Art
A piezoelectric material represented by a material having a lead-based perovskite structure such as PZT (Pb(lead) zirconate titanate) provides a piezoelectric effect that it expands and contracts when a voltage is applied thereto. A piezoelectric element having such a property is utilized in various uses such as ultrasonic transducers, piezoelectric actuators, and piezoelectric pumps.
The structure of a piezoelectric element is basically a single-layer structure in which electrodes are formed on both ends of one piezoelectric material. However, according to microfabrication and integration of piezoelectric elements with recent developments of MEMS (micro electro mechanical systems) related devices, multilayered piezoelectric elements each having plural piezoelectric materials and plural electrodes alternately stacked have been used. In such a piezoelectric element, the capacitance of the multilayered structure as a whole can be made larger by connecting electrodes for applying electric fields to the respective plural piezoelectric material layers in parallel. Accordingly, the rise in electrical impedance can be suppressed even when the size of the piezoelectric element is made smaller.
FIGS. 7A and 7B are sectional views for explanation of a method of manufacturing a conventional multilayered piezoelectric element. As shown in FIG. 7A, three piezoelectric material layers 10 and two layers of internal electrodes 21 and 22 are alternately stacked to form a multilayered structure. Further, side insulating films 23 are formed in predetermined positions on the side surfaces of the multilayered structure. Then, side electrodes 24 and 25 and an upper electrode 26 and a lower electrode 27 are formed by applying coating of an electrode material around the multilayered structure.
Then, using a dicing saw, parts “G” and “H” as shown in FIG. 7B are removed, and the side electrode 24 is separated from the lower electrode 26 and the side electrode 25 is separated from the upper electrode 27. Thereby, electric fields can be respectively applied to the three piezoelectric material layers 10 while the short-circuit between the lower electrode 26 and the upper electrode 27 is eliminated.
In this regard, as the frequency of ultrasonic waves transmitted and received by an ultrasonic probe is higher, it becomes necessary to make the thickness of the piezoelectric element smaller. Accordingly, it becomes necessary to make the widths of the side insulating films formed on the side surfaces of the multilayered structure smaller. As a method of forming the insulating films on the side surfaces of the multilayered structure, emulsion electrodeposition, photolithography, printing, and dispensing are used, but it becomes a problem whether or not those methods can deal with the need of making the width of the insulating films smaller.
In resin emulsion electrodeposition, generally, the diameter of emulsion particles is as large as about 50 μm or more, and there is a limit to the width of the insulating films, which can be made around 100 μm as the smallest. Glass emulsion electrodeposition is also proposed, however, if glass as a hard material is used as the insulating films, the electromechanical coupling factor “k” of the piezoelectric material becomes lower. For example, a comparison is made between electromechanical coupling factors k33 in the “33” vibration mode as a vibration mode in which, to a piezoelectric material poled in the third direction (Z-axis direction), an electric field is applied in the same third direction. When k33 of the piezoelectric material is 0.74, if a resin is used as the insulating films, k33 becomes about 0.68 or more, however, if glass is used as the insulating films, k33 becomes as low as about 0.60 to 0.62.
According to photolithography of exposing a resist film to light for patterning, a fine pattern on the order of submicron can be formed, however, it is difficult to form a thick resist film for the insulating films, and, if a thick resist film can be formed, the pattern may be rough. Further, in photolithography, there are problems that times taken for the respective steps are long, and the piezoelectric material is largely damaged because the piezoelectric elements are immersed in an alkaline solution and the resist material is baked at a high temperature in the developing step.
When screen printing is used, the precision of the plate is insufficient and screen alignment is difficult. Further, in formation of the insulating films by dispensing, principally, the lower limit of the width of the insulating films is determined by the size of the dispensing nozzle. In combination of the inner diameter of a dispensing nozzle that is currently and commercially available and the viscosity of an insulating material that can be applied, the limit to the width of the insulating films is about 90 μm to 100 μm. It is conceivable to use an insulating material having high viscosity, but that insulating material cannot be supplied stably from a dispensing nozzle having a small inner diameter.
As a related technology, Japanese Patent Application Publication JP-P2004-79825A discloses a method of manufacturing a multilayered piezoelectric ceramic element by which insulating layers can be formed at a low temperature of 500° C. or less. The manufacturing method is a method of manufacturing a multilayered piezoelectric ceramic element having an external electrode on an insulator insulating every other internal electrode, which is exposed on side surfaces of a multilayered structure having stacked three or more layers of piezoelectric ceramic and two or more layers of internal electrodes, so as to form respective one of opposite electrodes, and includes the step of forming the insulator by applying an insulating material processed to a paste state by using a dispenser.
Further, Japanese Patent Application Publication JP-P2004-111718A discloses a method of manufacturing a multilayered piezoelectric actuator that can sufficiently secure connection between internal electrodes and an external electrode and suppress insulation degradation and breakage due to the discharge phenomenon between the internal electrodes and another external electrode. The manufacturing method is a method of manufacturing a multilayered piezoelectric actuator of forming a multilayered structure by alternately stacking plural piezoelectric materials and plural internal electrodes and connecting every other internal electrode exposed on surfaces of the multilayered structure to the external electrode, and includes the step of forming a notch groove on every other exposed internal electrode, then forming an insulating layer on the side surface on which the internal electrodes are exposed so as to expose only ends of the internal electrodes having the same polarity as that of the external electrode on the surface, and forming the external electrode on the side surface.
However, higher frequencies are desired in ultrasonic probes, and an ultrasonic probe for transmitting and receiving ultrasonic waves at the center frequency of 10 MHz to 15 MHz is necessary to be created. For the purpose, the thickness of a piezoelectric element is 120 μm to 150 μm, and, in the case of a multilayered piezoelectric element having a two-layer or three-layer structure, the thickness of one layer is 40 μm to 75 μm. Accordingly, it is becoming difficult to form insulating films by dispensing. Further, it is also becoming difficult to form a groove in each exposed internal electrode as disclosed in JP-P2004-111718A.